To achieve high detection sensitivity in a mass spectrometry device, it is important for ions derived from sample components generated in an ion source to be fed into the mass spectrometer, such as a quadrupole mass filter, etc., as efficiently as possible. In particular, in mass spectrometry devices such as liquid chromatography-mass spectrometry device, where ionization is performed under atmospheric pressure, even under conditions of low vacuum atmosphere, i.e. when there are relatively many residual gas molecules, it is important to reduce the influence of scattering due to collision with such gas molecules as much as possible, and to transport ions to the mass spectrometer while minimizing losses. To achieve this objective, an ion optical element known as an ion guide is used for focusing the ions sent from the preceding stage and feeding them into the mass spectrometer, etc. of the next stage.
The general configuration of an ion guide is a multipole configuration in which 4, 6, 8 or more substantially round cylindrical rod electrodes are spaced apart from each other at the same angle and arranged in parallel to each other so as to surround the ion optical axis. In a multipole ion guide of this sort, normally, high frequency voltages of the same amplitude and frequency but of inverted phase are applied respectively to two rod electrodes adjacent in the circumferential direction about the ion optical axis. When this sort of high frequency voltage is applied to each rod electrode, pseudo-potential barriers are formed by the high frequency electric field generated between the electrodes, and ions are reflected between these potential barriers as they travel downstream. As a result, ions scattered due to collision with residual gas molecules can also be stably transported and the sensitivity of the device can be increased.
Quadrupole, hexapole and octupole configurations are commonly used for multipole ion guides. It is known that when the voltage applied to the rod electrodes is the same, the greater the number of poles, the greater the ion confinement potential in the vicinity of the rod electrodes. It is furthermore known that the ability to focus ions near the ion optical axis is higher when the number of poles is smaller. FIG. 11 is a drawing which schematically illustrates the relationship between radial distance r from the ion optical axis (center) and the confinement potential φ in a quadrupole ion guide and an octupole ion guide (see Patent literature 1, etc.).
It can be seen that in an octupole ion guide, the confinement potential rises sharply and the ion confinement capacity is higher at locations near the rod electrodes (away from the center). On the other hand, since the bottom of the potential well is wide, ions can be readily present not just near the ion optical axis but also at locations away from the optical axis. In other words, the degree of concentration of ions toward the vicinity of the ion optical axis is not particularly good. By contrast, with a quadrupole ion guide, the confinement potential rise is gradual, so the ion confinement capacity is relatively low, but the bottom of the potential well is limited to a narrow range in the vicinity of the ion optical axis, so ions are focused near the ion optical axis.
It will be noted that in a quadrupole ion guide, the confinement potential can be increased by increasing the amplitude of the high frequency voltage applied to each rod electrode, but a quadrupole ion guide has a low mass cutoff (LMC) limiting condition (see Patent literature 2, etc.), with the LMC increasing the more one raises the driving voltage. Thus, when driving voltage is raised in order to increase the confinement potential, the problem occurs that it becomes difficult to stably transport ions with a low mass-charge ratio, so there are limits to increasing the driving voltage.
Since the ion transport characteristics differ in this way between quadrupole ion guides and octupole ion guides, and also multipole ion guides with other numbers of poles, it is desirable to select an ion guides with the appropriate number of poles according to the conditions of use, such as the mass-charge ratio range of the ions to be analyzed. Specifically, when analyzing ions across a wide mass-charge ratio range, it is preferable to use to an octupole ion guide with high confinement capacity, and to detect ions with a specific mass-charge ratio or ions with a narrow mass-charge ratio range at high sensitivity, it is preferable to use a quadrupole ion guide, focus ions near the ion optical path and transport ions to the subsequent stage ion optical system at low loss. Because of this, in order to obtain good analysis results, it is desirable to be able to rapidly switch the effective number of poles of the multipole ion guide even during execution of liquid chromatography/mass spectrometry (LC/MS) or gas chromatography/mass spectrometry (GC/MS).
However, in conventional mass spectrometry devices, switching the effective number of poles as described above is difficult for the following reasons. Namely, the high frequency voltage applied to each rod electrode of the multipole ion guide requires an amplitude of approximately several hundred V, and to generate such a voltage, LC resonant circuits employing inductance and capacitance are generally used in the prior art. FIG. 10 is a simplified diagram showing the electrode configuration and driving circuit of a conventional octupole ion guide.
In FIG. 10, the eight rod electrodes 221 through 228 contained in ion guide electrode unit 200 are arranged so as to be inscribed into a virtual round cylindrical body P having the ion optical axis C at its center and so as to be spaced apart at equal angular intervals (45°) in the circumferential direction. Sets of four of these eight rod electrodes 221 through 228, consisting of every other one in the circumferential direction (rod electrodes 221, 223, 225 and 227; and rod electrodes 222, 224, 226 and 228) are electrically connected, and voltage from a power supply unit 500 is applied to each of these two electrode groups. Looking at the ion guide electrode unit 200 from the power supply unit 500, an electrostatic capacitance C′ exists between circumferentially adjacent rod electrodes, and this electrostatic capacitance C′ is connected in parallel to a variable capacitance capacitor 503 having a capacitance C. The LC resonant circuit, formed by this electrostatic capacitance C′ and capacitance C of variable capacitance capacitor 503 and the inductance L of coil 502, increases the amplitude of the high frequency signal inputted from high frequency signal generating unit 501, which is then applied to the rod electrodes 221 through 228. The resonant frequency is fixed, and the capacitance C of the variable capacitance capacitor 503 is adjusted to match the resonant frequency fLC of the LC resonant circuit to a specific frequency f.
In FIG. 10, if four electrode pair sets are formed taking two circumferentially adjacent rod electrodes as one set, and the electrical connection is switched by a switch such as an electromagnetic relay so that a high frequency voltage of reverse polarity is applied to circumferentially adjacent electrode pairs, a quadrupole electric field can be formed in the space surrounded by rod electrodes 221 through 228. That is, the effective number of poles can be switched from 8 to 4. However, when this sort of switching is performed, the electrostatic capacitance C′ between the rod electrodes changes, and thus the resonant frequency fLC of the LC resonant circuit deviates from the specific frequency f and adequate amplification of amplitude becomes impossible. In other words, high speed switching as described above was not possible because the capacitance C of variable capacitance capacitor 503 needs to be readjusted in response to change in electrostatic capacitance C′ between the rod electrodes in order to modify the effective number of poles. Furthermore, the switching itself was a very laborious operation and was not practical.
Additionally, to meet demands for enhanced sensitivity, enhanced accuracy or other improved qualities in the mass spectrometers, it is necessary to bring the shape of equipotential lines in the electric field in the ion guide closer to a theoretically-derived predetermined curve, thereby improving the qualities such as ion receiving properties and ion passing properties. To this end, the accuracy in the arrangement of the respective rod electrodes in the ion guide needs to be improved, and in order to achieve the improvement, the present applicant proposed an ion guide having a novel configuration in Patent Literature 3. One example of the ion guide is described with reference to FIG. 12 to FIG. 15.
FIG. 12A is a side view of an ion guide unit 100, and FIG. 12B and FIG. 12C are respectively sectional views on the lines A-A′ and B-B′ in FIG. 12A. The ion guide unit 100 includes an ion guide 110 in which eight metal plates extending in the direction of an ion optical axis C are employed as electrodes, and a hollow cylindrical case 140 that encloses the ion guide 110. The respective electrodes of the ion guide 110 are arranged rotationally symmetrical so as to be apart at an interval of an angle of 45° around the ion optical axis C, with their longitudinal-side end surfaces directed toward the ion optical axis C. Here, four electrodes alternately positioned among the eight electrodes are employed as first electrodes 111, and four electrodes adjacent thereto are employed as second electrodes 112.
FIG. 13 is a perspective view of one of the first electrodes 111. In the first electrode 111, an end edge on the side of the ion optical axis C has an arc shape or a hyperbolic shape bulging toward the ion optical axis C in a sectional plane perpendicular to the ion optical axis C. Further, the end edge on the side of the ion optical axis C is slightly inclined with respect to the ion optical axis C so as to become slightly apart from the ion optical axis C as an ion travels (rightward in FIG. 12C and FIG. 13). Because of the inclination, the intensity of the multipole electric field is smaller toward the outlet side of the ion guide 110, thereby decelerating flying ions. The other three plate electrodes of the first electrode 111, and the four electrode plates of the second electrodes 112 adjacent thereto also have the same shape.
The case 140 includes a tubular section 141 that encloses the first electrodes 111 and the second electrodes 112, a first support section 142 that is attached to one end portion of the tubular section 141 to support one end surfaces (left-side end surfaces in FIG. 12C) of the respective electrodes, a second support section 143 that is attached to the other end portion of the tubular section 141, and a disk spring fixing section 144 that fixes a disk spring 130 as shown in FIG. 14A by sandwiching the disk spring 130 between the disk spring fixing section 144 and the second support section 143. The first support section 142 and the second support section 143 are made of insulators such as ceramics, plastics or the like, and an opening for allowing ions to pass therethrough is provided in the center. A cylindrical through hole is provided in the second support section 143 at a position corresponding to each of the electrodes.
The disk spring 130 shown in FIG. 14A is made of metal, and includes a ring-shaped frame portion 131 and eight spring portions 132 working as cantilever springs projecting inward from the frame portion 131. The spring portions 132, each having a T shape with the head inward, are arranged so that the heads are close, but without contacting, to each other.
A thin plate 150 made of metal as shown in FIG. 14B is placed on a surface supporting the electrodes in the first support section 142. The thin plate 150 includes a ring-shaped frame portion 151 and four metal contacts 152 projecting inward from the frame portion 151. In the thin plate 150 placed on the first support section 142, the positions of the metal contacts 152 correspond to the positions of the first electrodes 111. Accordingly, the thin plate 150 contacts only the first electrodes 111, and does not contact the second electrodes 112.
FIG. 15 is a plan view of a state in which the disk spring 130 and the disk spring fixing section 144 are removed from the end portion on the side of the second support section 143 of the ion guide unit 100. Insulating spacers 121 made of insulators are inserted into four holes corresponding to the first electrodes 111, and conducting spacers 122 made of conductors are inserted into four holes corresponding to the second electrodes 112, as to the eight through holes provided in the second support section 143. The respective spacers are cylindrical members having the same length, which sets one end of the spacer to slightly project from the surface of the second support section 143 when the other end is in contact with the electrode.
FIG. 16 is a partial plan view of a state in which the disk spring 130 and the disk spring fixing section 144 are attached to the ion guide unit 100 shown in FIG. 15. The disk spring 130 is arranged such that the right and left ends close to each other of the adjacent spring portions 132 press the projecting portion of one insulating spacer 121 or one conducting spacer 122. Accordingly, the disk spring 130 is insulated from the first electrodes 111 by the insulating spacers 121, and electrically connected to the second electrodes 112 via the conducting spacers 122.
In the ion guide unit 100 having the above configuration, the spring portions 132 of the disk spring 130 press the first electrodes 111 and the second electrodes 112 toward the first support section 142 via the insulating spacers 121 or the conducting spacers 122. Accordingly, the respective electrodes 111 and 112 are sandwiched between the disk spring 130 and the first support section 142 from both sides and thereby fixed. At this point, end surfaces of the first electrodes 111 are in contact with the insulating spacers 121 or the metal thin plate 150, and end surfaces of the second electrodes 112 are in contact with the conducting spacers 122 or the second support section 143 made of an insulator. A voltage VDC+v·cos ωt in which a radio-frequency voltage v·cos ωt is superimposed on a direct current voltage VDC is applied to the first electrodes 111 via the thin plate 150, and a voltage VDC-v·cos ωt in which a radio-frequency voltage of inverted phase (i.e., phase shifted by 180°) is superimposed on the same direct current voltage is applied to the second electrodes 112 via the disk spring 130 and the conducting spacers 122 from a voltage application section (not shown in the drawing). Accordingly, a multipole radio-frequency electric field is formed in the space surrounded by the edge end surfaces of the eight electrodes 111 and 112, and ions introduced therein are converged.
Since the end edges of the eight electrodes 111 and 112 facing the ion optical axis C have an arc shape or a parabolic shape convex toward the ion optical axis C in a plane perpendicular to the ion optical axis C, an electric field whose equipotential lines are shaped along the curve is generated in the vicinity of the electrodes 111 and 112. Thus, an electric field nearly an ideal state can be formed in the space surrounded by the end surfaces of the respective electrodes 111 and 112.
Recent mass spectrometers tend to have complicated configurations where, for example, a plurality of multipole-type ion guides as described above are used. In a liquid-chromatograph tandem quadrupole mass spectrometer described in Non Patent Literature 1, for example, a two-stage octupole-type ion guides are provided between an ion source and a first-stage quadrupole mass filter, and a quadrupole-type ion guide is disposed within a collision cell. That is, a plurality of ion guides having different number of poles are used in an apparatus. In conventional mass spectrometers, ion guides having different number of poles as described above have respective configurations different from each other. For example, when the above ion guide unit 100 is used, it is necessary to change not only the number of the metal plate electrodes, but also the shape of the members for holding the metal plate electrodes, such as the first support section 142, the second support section 143 and the disk spring 130, according to the number of poles. If, in the mass spectrometer using a plurality of ion guides as described above, ion guides having the same structure can be used, it is advantageous in reducing the cost.